Within the field of cutting or chip-removing machining of, above all, workpieces of metal, drilling of holes is a frequent operation. Generally, drilling may be divided into two different main disciplines, viz. short-hole drilling and long-hole drilling, respectively. Although these two disciplines have certain features in common, they are in practice completely different in many essential respects. As an old rule of thumb, short-hole drilling generates hole depths that amount to at most 5× the diameter, while long-hole drilling generates hole depths from 5× the diameter up to 100× the diameter, or more. In comparison with long-hole drilling, short-hole drilling may be carried out by means of tools and machines that are fairly uncomplicated, inexpensive and smoothly re-adjustable, in order to be able to provide a large number holes per unit of time, e.g., in one and the same workpiece. Usually, the necessary supply of cooling liquid to the cutting inserts of the drill takes place internally through the drill body from the machine, while the chip evacuation out of the drilled hole takes place via external, open flutes, which may be straight or helicoidal. In such a way, the drill and the workpiece can be readjusted quickly in relation to each other while minimizing the set-up times. In this connection, it should also be mentioned that short-hole drilled holes have a length that in absolute numbers rarely exceeds 300 mm.
Long-hole drilling differs from short-hole drilling not only in respect of the simple fact that the recessed holes have a great depth, but above all, of the fact that the depth of the holes imposes completely different and stiffer requirements on the requisite equipment in the form of machines, drills and fittings than the comparatively moderate requirements imposed on equipment for simple short-hole drilling. Thus, long-hole drilling is normally carried out in special long-hole drilling works, which are complicated and space-demanding, among other things, in order to allow any set of drill and workpiece according to three different alternatives, viz.: only rotating workpiece, only rotating drilling tool, or rotation of both the workpiece and the drilling tool. In the work, in addition to powerful driving and axial feeding mechanisms, sophisticated devices for the supply of cooling liquid, centering of the drill upon engagement, etc., are also included. In the dominant drilling systems, viz. STS (Single Tube System) and the Ejector system, the proper drill includes a tube and a replaceable front head, which is provided with asymmetrically placed cutting inserts and formed with an internal, through hole or bore through which released chips can pass to the interior of the tube for forwarding rearwardly to a collection device. In other words, in this case the chip evacuation takes place internally through the drill head and the tube, while the cooling liquid is fed forward externally along the drill head, viz. in the boundary layer between the envelope surface of the drill head and the hole wall. In this connection, the cooling liquid also serves as a lubricant between the hole wall and the drill. In order for the drill head to move straight, i.e., without deflecting from the ideally straight line defined by the center axis of the drill, the drill head includes particular support pads, which have the purpose of supporting and guiding the head against the action of such radial forces that are generated as a consequence of the asymmetrical placement of the cutting inserts in relation to the center axis. Furthermore, additional support pads may be arranged remotely behind the drill head, viz. either on a particular adapter between the drill head and the tube, or on the proper tube. The principal task of the trailing set or sets of support pads is to equalize occurring irregularities in the surface of the hole wall with the purpose of giving the hole wall an optimized surface finish.
It should also be mentioned that the workpieces machined by long-hole drilling frequently are very expensive, and hence any cassations become disastrous in respect of economy. For instance, the workpieces may be blanks for rolls, propeller shafts, etc., the length of which often exceeds 10 m, and the value of which, in the finish-machined state, at times approaches the value of the equipment by means of which the machining is carried out.
Furthermore, it should be pointed out that long-hole drills rarely or never are used for short-hole drilling according to the aforementioned rule of thumb. However, the development of such drills that traditionally relate to the group of short-hole drills has now been driven so far that they can be used for making holes that are deeper than 5× the diameter.
The present invention relates to a short-hole drill of the type initially mentioned, which is exclusively suitable and intended for short-hole drilling. Like long-hole drills, also certain types of short-hole drills are made with asymmetrically placed cutting inserts, the shank of the drills having been made with a single pair of support pads located in the area of the front end of the drill, viz. peripherally in the immediate vicinity of the front-end surface in which the cutting inserts and the pockets thereof are located. Such short-hole drills fulfill, in a profitable way, the task of drilling continuous holes in solid workpieces, more precisely by so-called solid drilling, wherein an unbroken hole wall is formed between the opposite ends of the hole. However, if the drill is to pass one or more hollow spaces inside a workpiece, e.g., when drilling crossing holes, the drill may lose its guiding and deflect when it passes the hollow space. When the drill tip in the next step, after having passed the hollow space, again should enter the material of the workpiece, there is a risk that the entering takes place at a point that deviates from the geometrical center line that defines an optimally straight hole. In other words, the straightness and dimensional accuracy of the hole may be spoiled-something that in the worst case may lead to cassation of the workpiece.
As an example of a short-hole drill according to the prior art, JP 2004314258A may be mentioned, which describes how supporting pads may be provided in a drill body in the vicinity of asymmetrical cutting inserts mounted at the front end thereof. These support pads may be of different lengths, but they are all located in close proximity of the front end of the drill body.
The present invention aims at obviating the above-mentioned disadvantage of previously known short-hole drills and at providing an improved short-hole drill. Therefore, an object of the invention is to provide a short-hole drill, the tip or front end of which retains the desired centering also in connection with the same passing one or more hollow spaces inside a workpiece. In other words, the drill should generate optimally straight holes in spite of the hole wall being interrupted by transverse hollow spaces at one or more spots.